A standard mold for an injection-molding system has at least two mold parts or halves and even a slide that when fitted together form a closed compartment. A port formed in one of the mold parts opens into the compartment and serves for introduction of a fluent but hardenable synthetic resin into it. The mold parts and any slide that might be provided fit together at carefully machined edges that are perfectly complementary to each other so that the pressurized resin in the compartment cannot leak out between these edges.
In the system described in German patent 3,216,063 of Strunk a system is described for molding an annular plastic bead around the edge of a sheet of glass, as used for instance in a motor vehicle. To this end the mold parts have two pairs of annular ridges or edges, one within the other, that define an annular compartment. The outer pair of edges engage each other as in a standard mold, but the inner pair engage opposite faces of the workpiece directly across from each other.
Thus with this system the fit of the edges is extremely critical. Both pairs of edges must fit perfectly to prevent leakage from the annular compartment around the edge of the workpiece. With time, however, the outer pair of edges can wear so that the inner pair will grip the workpiece too tightly and possibly break it, or alternately the outer edges will not meet properly and a leak will occur. Thus it is standard, after a certain number of uses, to have to remachine the mold parts to reform the critical sealing edges.